Even in the era of anti-knock additives such as tetraethyl lead, the use of alkylate as a component in motor fuel gained both universal acceptance and importance. In the ensuing years alkylate has become an even more important component of motor fuel. Alkylate is an economical, clean-burning, high-octane, low volatility product that is becoming increasingly important as the composition of gasoline changes in response to environmental concerns and legislation. The governmental regulations most applicable to the increasing importance of alkylates are those affecting lead and butane. Adding lead anti-knock compounds was the easiest way to raise gasoline octane, but because of continuing concerns over the effects of lead emissions the phasing out of lead in gasoline was required, a process over 90% complete. Butane is another effective octane-booster but tends to evaporate from gasoline, especially in warm weather, contributing to smog formation. Recent EPA regulations have effected their virtually complete removal from gasoline.
The term "alkylate" generally refers to a complex mixture resulting from the alkylation of olefins present or formed in a feedstream of C2-C6 olefins with intermediates arising primarily from alkanes, especially branched alkanes, and predominantly those with 4 carbon atoms, especially isobutane, also present in the same feedstream. It is most desirable that the complex product mixture referred to as alkylate contains predominantly trimethylpentanes, since these are high-octane components which add considerable value to motor fuel, yet the chemistry of alkylation affords a dazzling variety of products resulting from only a few basic chemical reactions characteristic of the carbonium ion which plays a central role in the alkylation process. Thus, chain transfer (intermolecular hydride transfer and alkyl shifts), oligomerization and disproportionation serve to place into the alkylate as byproduct materials of from 5-12 +carbon atoms from a feed containing only C4 olefins and alkanes.
The alkylation of olefins is catalyzed by strong acids generally. Although such alkylation has been the focus of intense and continuing scrutiny for several decades, the requirements of optimum selectivity while achieving high conversion have heretofore narrowed, for all practical purposes, the commercial choice of catalyst to sulfuric acid and liquid hydrogen fluoride. While processes based on each of these acids have gained commercial acceptance those based on HF have been favored at least in part because of the relative ease of HF regeneration. A brief but valuable overview of HF-catalyzed alkylation is presented by B. R. Shah in "Handbook of Petroleum Refining Processes", R. A. Meyers, editor, McGraw-Hill Book Company, 1986, pp 1-3 through 1-28.
In a rather over-simplified description, the HF-catalyzed alkylation process is carried out as follows. Olefinic and isobutane feedstocks are combined and mixed with HF in an alkylation reaction zone. The reactor effluent is separated into the desired alkylate, acid, and light gases which are predominantly unreacted isobutane. The HF is either recycled to the reactor directly or regenerated, in whole or in part, prior to its being recycled to the reactor. Unreacted isobutane also is recycled to the reactor, and the alkylate is then used in motor fuel blending.
Recently HF (hydrofluoric acid) has come under environmental pressure. Hydrofluoric acid is classified as an Acutely Hazardous Material, and in Southern California the Board of the South Coast Air Quality Management District recently required that the use of HF in alkylation be phased out by Jan. 1, 1998. Consequently there is increasing reason to seek substitutes for HF as an alkylation catalyst for alkylate production. It is quite desirable to have a solid acid as an effective catalyst, for this permits development of fixed bed processes, a desirable alternative in the petroleum refining industry.
One of the promising solid catalysts for alkylation of C2-C6 olefins with alkanes in the 4 to 6 carbon range, a process hereafter specifically referred to as motor fuel alkylation, is the reaction product between one or more of the metal halides active as Friedel-Crafts catalysts and a refractory inorganic oxide having surface hydroxyl groups, where the refractory inorganic oxide also contains dispersed thereon a metal having hydrogenation activity for olefins. Such catalysts are reasonably well known in the art, as exemplified by U.S. Pat. No. 2,999,074, and includes, for example, the reaction product of aluminum chloride and alumina containing zerovalent platinum. As is commonly the case, these catalysts deactivate with use, where the deactivation is measured by the percent conversion of olefins, and it is imperative to have means of repeatedly regenerating these catalysts, i.e., to restore their activity, in order to utilize their catalytic effectiveness over long periods of time. It is further desirable that the method of regeneration be minimally disruptive to the motor fuel alkylation process itself. By that is meant that it is most desirable that the catalyst not be subjected to conditions or agents foreign to those of the alkylation process itself. It is still further desirable to minimize the regeneration cycle time relative to the alkylation cycle time. That is, if the complete process cycle time be the sum of the time during which the catalyst is used in alkylation (alkylation cycle time) and the time during which the catalyst is regenerated (regeneration cycle time) one desires that the latter be as short as possible. Of course the ideal regeneration cycle time is zero, but this corresponds to the case where the catalyst does not deactivate which, unfortunately, is contrary to experience.
Recently we developed a simple yet effective method of regenerating a deactivated catalyst of the aforedescribed type which satisfies both of the foregoing criteria; U.S. Pat. No. 5,310,713. More particularly, it was disclosed that after removing liquid hydrocarbons from the deactivated catalyst, treatment of the catalyst with hydrogen at approximately the same pressure as that used during alkylation and at reasonably low temperatures affords virtually complete regeneration, often with increased product quality. Our method is simple, very effective both in restoring activity and affording multiple regenerations, and requires a cycle time which is commercially feasible.
Subsequently, an improved catalyst was developed for alkylation where the catalyst was the reaction product of aluminum chloride and, the bound surface hydroxyl groups of, e.g., alumina containing a zerovalent metal having hydrogen activity and a metal cation, especially where the metal cation was one of the alkali or alkaline earth metal cations. Since the regeneration of a catalyst is a function of both the catalyst and the catalytic process causing its deactivation, and since regeneration often varies unpredictably for obscure reasons, or no apparent reason at all, we were gratified to find that the improved catalyst referred to above was regenerated by essentially the same process used for its predecessor catalyst.